"To date people have always described the sawfish as a sluggish bottom dweller," says Wueringer, who did the research as part of her PhD at the University of Queensland.

"Whereas, I've found that they're really agile and they are very, very fast at manipulating prey."

Once common in tropical and subtropical waters around the world, sawfish now have a more limited distribution.

The animals, which are related to sharks and rays, have a long snout with horizontally-projecting "teeth" around the edges of it and live on a diet of mullet, catfish and crustaceans.

Wueringer says it has been thought that sawfish live on the bottom, raking through the sand for prey, but she has found otherwise.

For up to four months she observed the behaviour of 19 sawfish that had just been captured from the wild.

The animals were freshwater sawfishes (Pristis microdon), which live in freshwater as juveniles and then move to the sea as they mature.

When the animals encountered prey in the water column, they lifted themself off the bottom and vigorously waved their saw around, says Wueringer.

They would knock the prey about, sometimes splitting it, and impaling it on their saw teeth.

Later the sawfish would go back to the bottom and wipe the prey off the saw.

When the animals encountered prey on the bottom, they were less likely to use lateral swipes of their saw, says Wueringer.

Instead, they would tend to move along the bottom with the prey pinned by the underside of their saw. This would enable the sawfish to turn the prey around to prevent scales and fins from getting caught in their gullet when they swallowed.

"It becomes really important when sawfish are eating catfish. Catfish can lock their fins in position and the fin spines are poisonous, and if you ingest them the wrong way round I think it could kill," says Wueringer.

Electro-receptors

All living organisms are surrounded by weak electric fields, which sharks and rays are able to detect using electro-receptors on their body.

These, together with tiny canals in the skin covering their saw that can detect water movements, give sawfish a hunting edge in the dark and murky waters where they live.

In her experiment, Wueringer used weak electric fields to mimic the presence of hidden prey.

When sawfish were presented with an electric field on the bottom, rather than rake through the sand, they just tried to bite the field.

When they were presented with an electric field in the water column, 20 to 30 centimetres above the bottom, they side swiped it with their saw.

Together with other experiments these findings suggest that the saw evolved to enable sawfish to capture prey in the water column, says Wueringer.

"Sawfish are endangered so it's important to get more information about them and what they do and about their behaviour," she says.

She says protection measures for an animal that sits and feeds on the bottom are different from those used to protect an animal that uses the three-dimensionality of the water.

Wueringer says the fact that sawfish use the water column means they can more easily get tangled in fishing nets than previously thought.

She says this new understanding could inform more effective conservation efforts.